JS: Fix Uncaught ReferenceError convertkit

[n7studios]

Summary

Fixes an uncaught ReferenceError due to bundling frontend JS into a single file, and the convertkit object not always being available i.e. when the user is in the WordPress Administration interface.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

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[ciccio-kit]

LGTM ✅

[ciccio-kit]

question: do we have any way to track these console errors? I'm only wondering whether or not we are losing visibility on potential issues where convertkit is not expected to be undefined

[n7studios]

question: do we have any way to track these console errors? I'm only wondering whether or not we are losing visibility on potential issues where convertkit is not expected to be undefined

Not that I'm aware of. End to end tests test the functionality of scripts to ensure they're performing as expected. This wasn't a blocking bug that caused issues, though.